Anisotropic cellulose nanofibril aerogels fabricated by directional stabilization and ambient drying for efficient solar evaporation

Anisotropic aerogels, mimicking natural porous materials such as wood and bone, always exhibit distinctive properties due to their aligned channels, which, however, generally require complicated ice templating configuration and costly freeze-drying. In this work, we develop a novel, facile, low-cost, and scalable method combining directional stabilization and ambient drying to prepare anisotropic aerogels. The frozen aqueous TEMPO-oxidized cellulose nanofibrils (TOCNFs) monolith embedded in a mold is directly immersed in ethanol/Fe3+ bath. The ice crystals directionally dissolve in ethanol, and simultaneously the TOCNFs are directionally stabilized due to the electrostatic complexation between their carboxyl groups and Fe3+ ions. The obtained anisotropic nanocellulose aerogel (NCA) after ambient drying exhibits an ordered porous structure, which is tunable by adjusting the contents of carboxyl groups on TOCNFs. The NCAs reveal distinct anisotropy, excellent mechanical compressibility in the radial direction, and high water-stability. To demonstrate their promising potentials for application in various advanced fields such as solar steam generation, the NCA is coated with polypyrrole (PPy) by in-situ polymerization to achieve excellent photothermal property but well maintain the original aligned channels of NCA, which endow the NCA@PPy with an efficient solar evaporation performance.